LOFT small breach study using RETRAN

LOFT L3-6 has been analysed using the RETRAN code. L3-6 simulated a 4 inch (2.4%) break in a cold leg of a main coolant loop of a commercial PWR. Only high-pressure emergency coolant injection was used and the primary circuit depressurised. The primary pumps were tripped just before the conclusion. Conditions in the primary circuit were largely controlled by the break flow and the RETRAN predictions in this area were very good. This was due partly to fair accuracy in the prediction of break flow and injection flow over most of transient and probably to the fact that flow homogeneity produced by the operation of the pumps suited RETRAN calculation. It is now considered that an analysis of the L3-5 experiment should be carried out. In L3-5 the pumps were tripped immediately after breach of circuit. The predictions for the secondary side of the steam generator were fairly accurate in the early stages of the transient but this was not maintained. It now seems clear that a non-equilibrium treatment of this area is required. (U.K.)